Oxocarbon anion

In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula C
x
On
y
for some integers x, y, and n.

The most common oxocarbon anions are carbonate, CO2−
3
, and oxalate, C
2
O2−
4
. There is however a large number of stable anions in this class, including several ones that have research or industrial use. There are also many unstable anions, like CO
2
and CO
4
, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO4−
4
, that have been the subject of theoretical studies but have yet to be observed.

Stable oxocarbon anions form salts with a large variety of cations. Unstable anions may persist in very rarefied gaseous state, such as in interstellar clouds. Most oxocarbon anions have corresponding moieties in organic chemistry, whose compounds are usually esters. Thus, for example, the oxalate moiety [–O–(C=O)2–O–] occurs in the ester dimethyl oxalate H3C–O–(C=O)2–O–CH3.

Chemfm mellitate 6neg
2D diagram of mellitate C
12
O6−
12
, one of the oxocarbon anions. Black circles are carbon atoms, red circles are oxygen atoms. Each blue halo represents one half of a negative charge.

Distributed charges and resonances

In many oxocarbon anions each of the extra electrons responsible for the negative electric charges behaves as if it were distributed over several atoms. Some of the electron pairs responsible for the covalent bonds also behave as if they were delocalized. These phenomena are often explained as a resonance between two or more conventional molecular structures that differ on the location of those charges and bonds. The carbonate ion, for example, is considered to have an "average" of three different structures:

Carbonate-ion-resonance-2D
Delocalisation and partial charges on the carbonate ion
Space-filling model of the carbonate ion

Carbonate-ion-resonance-2D
Delocalisation and partial charges on the carbonate ion
Space-filling model of the carbonate ion

so that each oxygen has the same negative charge equivalent to ​23 of an electron, and each C–O bond has the same average valence of ​43. This model accounts for the observed threefold symmetry of the anion.

Similarly, in a deprotonated carboxyl group –CO
2
, each oxygen is often assumed to have a charge of −​12 and each C–O bond to have valence ​32, so the two oxygens are equivalent. The croconate anion C
5
O2−
5
also has fivefold symmetry, that can be explained as the superposition of five states leading to a charge of −​25 on each oxygen. These resonances are believed to contribute to the stability of the anions.

Related compounds

Oxocarbon acids

An oxocarbon anion C
x
On
y
can be seen as the result of removing all protons from a corresponding acid CxHnOy. Carbonate CO2−
3
, for example, can be seen as the anion of carbonic acid H2CO3. Sometimes the "acid" is actually an alcohol or other species; this is the case, for example, of acetylenediolate C
2
O2−
2
that would yield acetylenediol C2H2O2. However, the anion is often more stable than the acid (as is the case for carbonate);[1] and sometimes the acid is unknown or is expected to be extremely unstable (as is the case of methanetetracarboxylate C(COO)4).

Neutralized species

Every oxocarbon anion C
x
On
y
can be matched in principle to the electrically neutral (or oxidized) variant CxOy, an oxocarbon (oxide of carbon) with the same composition and structure except for the negative charge. As a rule, however, these neutral oxocarbons are less stable than the corresponding anions. Thus, for example, the stable carbonate anion corresponds to the extremely unstable neutral carbon trioxide CO3;[2] oxalate C
2
O2−
4
correspond to the even less stable 1,2-dioxetanedione C2O4;[3] and the stable croconate anion C
5
O2−
5
corresponds to the neutral cyclopentanepentone C5O5, which has been detected only in trace amounts.[4]

Reduced variants

Conversely, some oxocarbon anions can be reduced to yield other anions with the same structural formula but greater negative charge. Thus rhodizonate C
6
O2−
6
can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C
6
O4−
6
and then to benzenehexolate C
6
O6−
6
.[5]

Acid anhydrides

An oxocarbon anion C
x
On
y
can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula CxOy−​n2; namely, the acid minus ​n2 water molecules H2O. The standard example is the connection between carbonate CO2−
3
and carbon dioxide CO2. The correspondence is not always well-defined since there may be several ways of performing this formal dehydration, including joining two or more anions to make an oligomer or polymer. Unlike neutralization, this formal dehydration sometimes yields fairly stable oxocarbons, such as mellitic anhydride C12O9 from mellitate C
12
O6−
12
via mellitic acid C12H6O12[6][7][8]

Hydrogenated anions

For each oxocarbon anion C
x
On
y
there are in principle n−1 partially hydrogenated anions with formulas H
k
C
x
O(nk)−
y
, where k ranges from 1 to n−1. These anions are generally indicated by the prefixes "hydrogen"-, "dihydrogen"-, "trihydrogen"-, etc. Some of them, however, have special names: hydrogencarbonate HCO
3
is commonly called bicarbonate, and hydrogenoxalate HC
2
O
4
is known as binoxalate.

The hydrogenated anions may be stable even if the fully protonated acid is not (as is the case of bicarbonate).

Extreme cases

The carbide anions, such as acetylide C2−
2
and methanide C4−, could be seen as extreme cases of oxocarbon anions C
x
On
y
, with y equal to zero. The same could be said of oxygen-only anions such as oxide O2−, superoxide O
2
, peroxide, O2−
2
, and ozonide O
3
.

List of oxocarbon anions

Here is an incomplete list of the known or conjectured oxocarbon anions

Diagram Formula Name Acid Anhydride Neutralized
Chemfm carbonite 2neg.svg :CO2−
2
carbonite HCO2H CO CO2
Chemfm carbonate 2neg.svg CO2−
3
carbonate CH2O3 CO2 CO3
Chemfm peroxocarbonate 2neg.svg CO2−
4
peroxocarbonate CO3 CO4
Chemfm orthocarbonate 4neg.svg CO4−
4
orthocarbonate C(OH)4 methanetetrol CO2 CO4
Chemfm acetylene dioxide 2neg.svg C
2
O2−
2
acetylenediolate C2H2O2 acetylenediol C2O2
Chemfm oxalate 2neg.svg C
2
O2−
4
oxalate C2H2O4 C2O3, C4O6 C2O4
Chemfm dicarbonate 2neg.svg C
2
O2−
5
dicarbonate C2H2O5 C2O4
Chemfm peroxodicarbonate 2neg.svg C
2
O2−
6
peroxodicarbonate
Chemfm cyclopropanetrione 2neg.svg C
3
O2−
3
deltate C3O(OH)2 C3O3
Chemfm mesoxalate 2neg.svg C
3
O2−
5
mesoxalate C3H2O5
Chemfm acetylenedicarboxylate 2neg.svg C
4
O2−
4
acetylenedicarboxylate C4H2O4
Chemfm cyclobutanetetrone 2neg.svg C
4
O2−
4
squarate C4O2(OH)2 C4O4
Chemfm dioxosuccinate 2neg.svg C
4
O2−
6
dioxosuccinate C4H2O6
Chemfm cyclopentanepentone 2neg.svg C
5
O2−
5
croconate C5O3(OH)2 C5O5
Chemfm methanetetracarboxylate 4neg.svg C
5
O4−
8
methanetetracarboxylate C5H4O8
Chemfm cyclohexanehexone 2neg.svg C
6
O2−
6
rhodizonate C4O4(COH)2 C6O6
Chemfm cyclohexanehexone 4neg.svg C
6
O4−
6
benzoquinonetetraolate; THBQ anion (CO)2(COH)4 THBQ C6O6
Chemfm cyclohexanehexone 6neg.svg C
6
O6−
6
benzenehexolate C6(OH)6 benzenehexol C6O6
Chemfm ethylenetetracarboxylate 4neg.svg C
6
O4−
8
ethylenetetracarboxylate C6H4O8 C6O6
Chemfm furantetracarboxylate 4neg C
8
O4−
9
furantetracarboxylate C8H4O9
Chemfm benzoquinonetetracarboxylate 4neg C
10
O4−
10
benzoquinonetetracarboxylate C
10
H
4
O
10
C
10
O
8
Chemfm mellitate 6neg C
12
O6−
12
mellitate C6(COOH)6 C12O9

Several other oxocarbon anions have been detected in trace amounts, such as C
6
O
6
, a singly ionized version of rhodizonate.[9]

See also

References

  1. ^ "Infrared and mass spectral studies of proton irradiated H2O + CO2 ice: evidence for carbonic acid", by Moore, M. H.; Khanna, R. K.
  2. ^ DeMore W. B.; Jacobsen C. W. (1969). "Formation of carbon trioxide in the photolysis of ozone in liquid carbon dioxide". Journal of Physical Chemistry. 73 (9): 2935–2938. doi:10.1021/j100843a026.
  3. ^ Herman F. Cordes; Herbert P. Richter; Carl A. Heller (1969). "Mass spectrometric evidence for the existence of 1,2-dioxetanedione (carbon dioxide dimer). Chemiluminescent intermediate". J. Am. Chem. Soc. 91 (25): 7209. doi:10.1021/ja01053a065.
  4. ^ Schröder, Detlef; Schwarz, Helmut; Dua, Suresh; Blanksby, Stephen J.; Bowie, John H. (May 1999). "Mass spectrometric studies of the oxocarbons CnOn (n = 3–6)". International Journal of Mass Spectrometry. 188 (1–2): 17–25. Bibcode:1999IJMSp.188...17S. doi:10.1016/S1387-3806(98)14208-2.
  5. ^ Haiyan Chen, Michel Armand, Matthieu Courty, Meng Jiang, Clare P. Grey, Franck Dolhem, Jean-Marie Tarascon, and Philippe Poizot (2009), "Lithium Salt of Tetrahydroxybenzoquinone: Toward the Development of a Sustainable Li-Ion Battery" J. Am. Chem. Soc., 131(25), pp. 8984–8988 doi:10.1021/ja9024897
  6. ^ J. Liebig, F. Wöhler (1830), "Ueber die Zusammensetzung der Honigsteinsäure" Poggendorfs Annalen der Physik und Chemie, vol. 94, Issue 2, pp.161–164. Online version accessed on 2009-07-08.
  7. ^ Meyer H, Steiner K (1913). "Über ein neues Kohlenoxyd C12O9 (A new carbon oxide C12O9)". Berichte der Deutschen Chemischen Gesellschaft. 46: 813–815. doi:10.1002/cber.191304601105.
  8. ^ Hans Meyer; Karl Steiner (1913). "Über ein neues Kohlenoxyd C12O9". Berichte der Deutschen Chemischen Gesellschaft. 46: 813–815. doi:10.1002/cber.191304601105.
  9. ^ Richard B. Wyrwas and Caroline Chick Jarrold (2006), "Production of C
    6
    O
    6
    from Oligomerization of CO on Molybdenum Anions". J. Am. Chem. Soc. volume 128 issue 42, pages 13688–13689. doi:10.1021/ja0643927
1,2-Bis(dicyanomethylene)squarate

1,2-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C10N4O2−2 or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C4O2−4 through the replacement of two adjacent oxygen atoms by dicyanomethylene groups =C(−C≡N)2.

The anion can be obtained by reacting squaric acid with n-butanol to obtain the diester 1,2-dibutyl squarate (an oily orange liquid) and treating the latter with metallic sodium and malononitrile (N≡C−)2CH2 to give the trihydrated disodium salt 2Na+·C10N4O2−2·3H2O, a yellow water-soluble solid. The hydrated salt loses the water below 100 °C, but the resulting anhydrous salt is stable up to 400 °C. Reaction of the sodium salt with the chlorides of other cations in ethanol affords the following salts:

dipotassium 2K+·K2C10N4O2−2, anhydrous, yellow, stable to 300 °C

dirubidium 2Rb+·Rb2C10N4O2−2, anhydrous, brown, stable to 300 °C

magnesium sodium chloride, Mg2+·Na+·Cl−·C10N4O2−2·​4 1⁄2H2O, dark yellow, dehydrates at 60–106 °C, stable to 461 °C

calcium disodium, 2Na+·Ca2+·2C10N4O2−2·9H2O, yellow, dehydrates at 50–90 °C, stable to 178 °C

barium, Ba2+·C10N4O2−2·2H2O, yellow, dehydrates at 87 °C, stable to 337 °C

tetra-n-butylammonium, 2(C4H5)4N+·C10N4O2−2·H2O, yellow, dehydrates at 145 °C, stable to 323 °CNuclear magnetic resonance shows that the aromatic character of the squarate core is retained.

1,3-Bis(dicyanomethylene)squarate

1,3-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C10N4O2−2 or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C4O2−4 through the replacement of two opposite oxygen atoms by dicyanomethylene groups =C(−C≡N)2.

The anion can be obtained by reacting squaric acid with aniline to form the diester 1,3-dianiline squarate (a yellow solid), before treating the diester with malononitrile (N≡C−)2CH2 and sodium ethoxide to give the disodium tetrahydrate salt 2Na+·C10N4O2−2·4H2O, an orange water-soluble solid. The hydrated salt loses the water below 100 °C, but the resulting anhydrous salt is stable up to 400 °C. Reaction of the sodium salt with salts of other cations in ethanol affords the following salts:

dipotassium sodium chloride 2K+·Na+·Cl−·K2C10N4O2−2·​1⁄2CH3CN, orange

rubidium sodium chloride 7Rb+·Na+·2Cl−·3Rb2C10N4O2−2·CH3CH2OH, orange, loses 1 ethanol at 96 °C, stable to 361 °C

magnesium disodium nitrate, Mg2+·2Na+C10N4O2−2·NO−3·6H2O·CH3CH2OH, orange, loses 1 ethanol and 6 H2O at 78 °C, stable to 482 °C

calcium, Ca2+·C10N4O2−2·6H2O, purple, dehydrates at 63–102 °C, stable to 468 °C

barium, Ba2+·C10N4O2−2·4H2O, orange, dehydrates at 71–96 °C, stable to 457 °C

tetra-n-butylammonium sodium, 2(C4H9)4N+·2Na+·2Cl−·2C10N4O2−2·CH3CH2OH, orange, loses 1 ethanol and 2 tetrabutylammonium at 111 °C, stable to 238 °CNuclear magnetic resonance shows that the aromatic character of the squarate core is retained.

2-(Dicyanomethylene)croconate

2-(Dicyanomethylene)croconate is a divalent anion with chemical formula C8N2O2−4 or ((N≡C−)2C=)(C5O4)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of one oxygen atom by a dicyanomethylene group =C(−C≡N)2.

The anion was synthesized and characterized by A. Fatiadi in 1980, by hydrolysis of croconate violet treated with potassium hydroxide. It gives an orange solution in water.

Acetylenedicarboxylic acid

Acetylenedicarboxylic acid or butynedioic acid is an organic compound (a dicarboxylic acid) with the formula C4H2O4 or HO2CC≡CCO2H. It is a crystalline solid that is soluble in diethyl ether.

The removal of two protons yields the acetylenedicarboxylate dianion C4O2−4, which consists only of carbon and oxygen, making it an oxocarbon anion. Partial ionization yields the monovalent hydrogenacetylenedicarboxylate anion HC4O−4.

The acid was first described in 1877 by Polish chemist Ernest Bandrowski. It can be obtained by treating α,β-dibromosuccinic acid with potassium hydroxide KOH in methanol or ethanol. The reaction yields potassium bromide and potassium acetylenedicarboxylate. The salts are separated and the latter is treated with sulfuric acid.Acetylenedicarboxylic acid is used in the synthesis of dimethyl acetylenedicarboxylate, an important laboratory reagent. The acid is commonly traded as a laboratory chemical. It can also be reacted with sulfur tetrafluoride to produce hexafluoro-2-butyne, a powerful dienophile for use in Diels-Alder reactions.

Carbonate

In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−3. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.

The term is also used as a verb, to describe carbonation: the process of raising the concentrations of carbonate and bicarbonate ions in water to produce carbonated water and other carbonated beverages – either by the addition of carbon dioxide gas under pressure, or by dissolving carbonate or bicarbonate salts into the water.

In geology and mineralogy, the term "carbonate" can refer both to carbonate minerals and carbonate rock (which is made of chiefly carbonate minerals), and both are dominated by the carbonate ion, CO2−3. Carbonate minerals are extremely varied and ubiquitous in chemically precipitated sedimentary rock. The most common are calcite or calcium carbonate, CaCO3, the chief constituent of limestone (as well as the main component of mollusc shells and coral skeletons); dolomite, a calcium-magnesium carbonate CaMg(CO3)2; and siderite, or iron(II) carbonate, FeCO3, an important iron ore. Sodium carbonate ("soda" or "natron") and potassium carbonate ("potash") have been used since antiquity for cleaning and preservation, as well as for the manufacture of glass. Carbonates are widely used in industry, e.g. in iron smelting, as a raw material for Portland cement and lime manufacture, in the composition of ceramic glazes, and more.

Croconate blue

Croconate blue or 1,2,3-tris(dicyanomethylene)croconate is a divalent anion with chemical formula C14N6O2−2 or ((N≡C−)2C=)3(C5O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of three oxygen atoms by dicyanomethylene groups =C(−C≡N)2. The term Croconate Blue as a dye name specifically refers to the dipotassium salt K2C14N6O2.

Croconate violet

Croconate violet or 1,3-bis(dicyanomethylene)croconate is a divalent anion with chemical formula C11N4O2−3 or ((N≡C−)2C=)2(C5O3)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of two oxygen atoms by dicyanomethylene groups =C(−C≡N)2. Its systematic name is 3,5-bis(dicyanomethylene)-1,2,4-trionate. The term croconate violet as a dye name specifically refers to the dipotassium salt K2C11N4O3.

Methanetetracarboxylate

In chemistry, methanetetracarboxylate is a tetravalent anion with formula C5O4−8 or C(COO−)4. It has four carboxylate groups attached to a central carbon atom; so it has the same carbon backbone as neopentane. It is an oxocarbon anion, that is, consists only of carbon and oxygen.

The term is also used for any salt with that anion; or for any ester with the C(COO)4 moiety.The salts and esters are relatively uncommon, and their uses appear to be limited to chemical research. The sodium salt Na4C(COO)4 can be obtained by oxidation of pentaerythritol C(CH2OH)4 with oxygen in sodium hydroxide solution at pH 10 and about 60 °C, in the presence of palladium as a catalyst. The tetraethyl ester C(COO-C2H5)4 is traded as a specialty chemical and has been used in organic synthesis.The anion can be seen as the result of removing four protons from methanetetracarboxylic acid, a hypothetical organic compound with formula C5H4O8 or C(COOH)4. However this acid has not been synthesised (as of 2009), and is believed to be unstable.

Orthocarbonic acid

hitler acid (methanetetrol) is the name given to a hypothetical compound with the chemical formula H4CO4 or C(OH)4. Its molecular structure consists of a single carbon atom bonded to four hydroxy groups. It would be therefore a fourfold alcohol. In theory it could lose four protons to give the hypothetical oxocarbon anion CO4−4 (orthocarbonate), and is therefore considered an oxoacid of carbon.

Orthocarbonic acid is highly unstable. Calculations show that it decomposes spontaneously into carbonic acid and H2O:

H4CO4 → H2CO3 + H2O.Orthocarbonic acid is one of the group of carboxylic ortho acids that have the general structure of RC(OH)3.The term ortho acid is also used to refer to the most hydroxylated acid in a set of oxoacids.

When drawn in two dimensions, the molecule resembles a swastika, except when we draw only hydroxy groups and the carbon is only as center, and has therefore been called "Hitler's acid".Researchers at the Moscow Institute of Physics and Technology predict that orthocarbonic acid is stable at high pressure; hence it may form in the interior of planets.

Oxocarbenium

An oxocarbenium ion (or oxacarbenium ion) is a chemical species characterized by a central sp2-hybridized carbon, an oxygen substituent, and an overall positive charge that is delocalized between the central carbon and oxygen atoms. A oxocarbenium ion is represented by two limiting resonance structures, one in the form of a carbenium ion with the positive charge on carbon and the other in the form of an oxonium species with the formal charge on oxygen. As a resonance hybrid, the true structure somewhere between the two. Compared to neutral carbonyl compounds like ketones or esters, the carbenium ion form is a larger contributor to the structure. They are common reactive intermediates in the hydrolysis of glycosidic bonds, and are a commonly used strategy for chemical glycosylation. These ions have since been proposed as reactive intermediates in a wide range of chemical transformations, and have been utilized in the total synthesis of several natural products. In addition, they commonly appear in mechanisms of enzyme-catalyzed biosynthesis and hydrolysis of carbohydrates in nature. Anthocyanins are natural flavylium dyes, which are stabilized oxocarbenium compounds. Anthocyanins are responsible for the colors of a wide variety of common flowers such as pansies and edible plants such as eggplant and blueberry.

Oxocarbon

An oxocarbon or oxide of carbon is a chemical compound consisting only of carbon and oxygen.The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (CO2) with IUPAC names carbon(II) oxide and carbon(IV) oxide respectively. Many other stable (practically if not thermodynamically) or metastable oxides of carbon are known, but they are rarely encountered, such as carbon suboxide (C3O2 or O=C=C=C=O) and mellitic anhydride (C12O9).

While textbooks will often list only the first three, and rarely the fourth, a large number of other oxides are known today, most of them synthesized since the 1960s. Some of these new oxides are stable at room temperature. Some are metastable or stable only at very low temperatures, but decompose to simpler oxocarbons when warmed. Many are inherently unstable and can be observed only momentarily as intermediates in chemical reactions or are so reactive that they can exist only in the gas phase or under matrix isolation conditions.

The inventory of oxocarbons appears to be steadily growing. The existence of graphene oxide and of other stable polymeric carbon oxides with unbounded molecular structures suggests that many more remain to be discovered.

Peroxycarbonate

In chemistry, peroxycarbonate (sometimes peroxocarbonate) is a divalent anion with formula CO2−4. It is an oxocarbon anion that consists solely of carbon and oxygen. It would be the anion of a hypothetical peroxocarbonic acid HO–CO–O–OH or the real hydroperoxyformic acid, HO-O-CO-OH (a.k.a. percarbonic acid, carbonoperoxoic acid).

The peroxycarbonate anion is formed, together with peroxydicarbonate C2O2−6, at the negative electrode during electrolysis of molten lithium carbonate. Lithium peroxycarbonate can be produced also by combining carbon dioxide CO2 with lithium hydroxide in concentrated hydrogen peroxide H2O2 at −10 °C.The peroxycarbonate anion has been proposed as an intermediate to explain the catalytic effect of CO2 on the oxidation of organic compounds by O2.The potassium and rubidium salts of the monovalent hydrogenperoxocarbonate anion H–O–O–CO−2 have also been obtained.

Pseudo-oxocarbon anion

In chemistry, the term pseudo-oxocarbon anion is used to refer to a negative ion that is conceptually derived from an oxocarbon anion through replacement of one or more of the basic oxygen atoms by chemically similar elements or functional groups, such as sulfur (S), selenium (Se), or dicyanomethylene (=C(CN)2).

Typical examples are the anions 2-(Dicyanomethylene)croconate, croconate violet, and croconate blue, derived from the croconate anion C5O2−5 by replacing one, two, or three oxygen atoms by dicyanomethylene groups:

These anions retain many of the properties of the parent, including the delocalized bond in the ring and the delocalized charge in the atoms attached to the ring. Similar anions can be obtained from squarate C4O2−4.

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